In an internal combustion engine, it is sometimes necessary to heat the fuel that is to be supplied to a fuel injection device such as a fuel injection valve and carburetor to ensure a reliable startup of the engine. For this purpose, various fuel heating devices have been proposed. Typically, an electric heating device is provided in a fuel supply pipe that leads to the fuel injection device to allow the heat of the electric heating device to be transferred to the fuel flowing along the surface of the heat emitting portion of the electric heating device, but the heat may not be uniformly transferred to the fuel, and a significant amount of cold fuel may be supplied to the fuel injection device depending on the operating condition of the engine. In such a case, the engine may not start up in an adequately stable manner.
Japanese patent laid open publication No. 05-26130 discloses a fuel heating device in which a heater having an elongated heat emitting portion is provided in a heating region defined immediately upstream of a fuel injection valve, and the fuel is allowed to flow along the length of the heat emitting portion of the heater before being supplied to the fuel injection valve.
However, according to this previous proposal, the heat provided by the heater may cause a part of the fuel to be vaporized, and the heater may be exposed to the vapor phase of the fuel. As a result, the fuel may not be efficiently heated, and the fuel may fail to be properly pressurized owing to the presence of the fuel vapor. The exposure of the heater to the vapor phase may even cause a premature failure of the heating wire in the heater.
Internal combustion engine, in particular for automotive uses, are typically provided with a plurality of cylinders. A fuel injection valve is provided for each cylinder, and a fuel delivery pipe having a circular cross section distributes the fuel pressurized by a fuel pump to the different fuel injection valves. The fuel injection valves are typically provided along the length of the fuel delivery pipe, and this may cause an unevenness in the amounts of fuel that the different fuel injection valves receive.
In the arrangement disclosed in WO2003/008796, the number of fuel injection valves serviced by each delivery pipe is reduced so that the unevenness may be minimized. However, this does not entirely solve the problem that the fuel injection valve connected to a downstream end of the fuel delivery pipe receives less fuel than the fuel injection valve connected to an upstream end of the fuel delivery pipe. This problem is particularly acute when a fuel heating unit is provided on the upstream end of each fuel injection valve.
In a fuel heating device provided with a plurality of heating units that correspond to different cylinders of a multiple cylinder engine, fuel is distributed to the different heating units via a horizontally extending fuel distribution pipe or other forms of conduits. The fuel distribution pipe is normally filled with liquid fuel, but the activation of the heating units may cause fuel vapor to migrate into the fuel distribution pipe. In such a case, a mixture of liquid fuel and vapor fuel coexist in the fuel distribution pipe where the liquid fuel fills the lower part of the fuel distribution pipe while the fuel vapor occupies a space above the upper surface of the liquid fuel.
In such a state, if the vehicle is subjected to an acceleration due to various vehicle motions such as cornering, accelerating and decelerating movements, or if the road surface is inclined with respect to a horizontal plane; the upper surface of the liquid fuel tilts in a corresponding manner, and this may prevent proper distribution of fuel to some of the heating units.
To avoid this problem, Japanese UM laid open publication No. 01-74361 proposes to form a downwardly projecting bulge in the part of the fuel distribution pipe adjacent to the inlet end of each fuel injection valve so that the liquid fuel may be available for each fuel injection valve even when the upper surface of the liquid fuel tilts with respect to the axial line of the fuel distribution pipe.
However, according to this previous proposal, the warm part of the fuel rises upward so that the lower part of each downwardly projecting bulge is normally occupied by the cold part of the fuel. Therefore, the cold fuel, instead of the properly heated fuel, may be preferentially supplied to the fuel injection valve.
Furthermore, when a horizontally extending fuel distribution pipe is used, and the vehicle is subjected to an acceleration such as when the vehicle is undergoing a cornering, accelerating or decelerating movement or the road surface is slanted with respect to a horizontal plane, the fuel distribution pipe is tilted from a horizontal orientation, and the warmer fuel is collected in the higher end of the fuel distribution pipe while the colder fuel is collected in the lower end of the fuel distribution pipe. As a result, the temperature of the fuel supplied to each fuel injection valve may varying from one fuel injection valve to another, and this may cause an unstable operation of the engine.